Cold sintering ceramics and composites

ABSTRACT

Cold sintering of materials includes using a process of combining at least one inorganic compound, e.g., ceramic, in particle form with a solvent that can partially solubilize the inorganic compound to form a mixture; and applying pressure and a low temperature to the mixture to evaporate the solvent and densify the at least one inorganic compound to form sintered materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/905,236, which is a divisional application ofU.S. patent application Ser. No. 15/277,553, which is a continuation ofInternational Application No. PCT/US2016/053772, filed Sep. 26, 2016,which claims the benefit of U.S. Provisional Application No. 62/234,389filed Sep. 29, 2015 and U.S. Provisional Application No. 62/349,444filed Jun. 13, 2016 the entire disclosures of each of which are herebyincorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant No.IIP1361571, awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

TECHNICAL FIELD

The present disclosure relates to sintering inorganic compounds, e.g.,ceramics, with or without other substances, at low temperatures.

BACKGROUND

Many ceramics and composites are sintered to reduce porosity and toenhance properties of the material such as strength, electricalconductivity, translucency, thermal conductivity, and heat capacity.However, many sintering processes involve the application of hightemperatures, typically above 1,000° C., to densify and improve theproperties of the material. The use of high temperatures precludes thefabrication of certain types of materials and adds to the expense offabricating the material or devices. In addition, co-sintering ofmulti-material systems is difficult due to the differences in thermalstability, shrinkage temperature onsets and rates, and the physical andchemical incompatibilities of the components at high temperatures.

Certain low temperature processes for sintering ceramic are known andcan address some of the issues related to high temperature sintering.For example, Ultra Low Temperature Cofired Ceramics (ULTCC) are firedbetween 450° C. and 750° C. See for example, He et al., “Low-TemperatureSintering Li₂MoO₄/Ni_(0.5)Zn_(0.5)Fe₂O₄ Magneto-Dielectric Compositesfor High-Frequency Application”, J. Am. Ceram. Soc. 2014:97(8):1-5. AlsoKahari et al. describe improving the dielectric properties of Li₂MoO₄ bymoistening water-soluble Li₂MoO₄ powder, compressing it, and postprocessing the samples at 120° C. See Kahari et al., J. Am. Ceram. Soc.2015:98(3):687-689. Kahari discloses the particle size of its powderswere less than 180 microns but that smaller particle sizes complicatesthe even moistening of the powders resulting in clay-like clusters,non-uniform density, warpage and cracking and that a large particle sizeis advantageous. Still others prepare ceramics by combining reactioncomponents to synthesize the ceramic at low temperatures. See, e.g.,U.S. Pat. No. 8,313,802. Such preparations take long periods of timelasing several hours to days to produce dense ceramics.

However, a continuing need exists for low temperature processes forsintering ceramics and composites.

SUMMARY OF THE DISCLOSURE

An advantage of the present disclosure is a process to densify materialsusing a solvent and at a temperature around the boiling point of thesolvent which is well below typical temperatures to sinter a material bymany hundreds of degrees centigrade. Advantageously, the processes ofthe present disclosure can use an aqueous based solvent and temperaturesno more than 200° C. above the boiling point of the solvent.

These and other advantages are satisfied, at least in part, by a processfor preparing a sintered material by combining at least one inorganiccompound in particle form with a solvent that can partially solubilizethe inorganic compound to form a mixture; and applying pressure and heatto the mixture to evaporate the solvent and densify the at least oneinorganic compound to form the sintered material. Advantageously, theapplied heat is at a temperature of no more than 200° C. above theboiling point of the solvent. The cold sintering process of the presentdisclosure can advantageously provide dense materials in short periodsof time.

Another aspect of the present disclosure includes a process forpreparing a sintered composite, the process comprising combining atleast one inorganic compound in particle form and at least one othersubstance with a solvent that can partially solubilize the inorganiccompound to form a mixture; and applying pressure and heat to themixture to evaporate the solvent and densify the at least one inorganiccompound to form the composite. Advantageously, the applied heat is at atemperature of no more than 200° C. above the boiling point of thesolvent. The other substance can be a different inorganic compound or itcan be a polymer, metal or other material such as a glass or carbonfibers, for example. Advantageously, the low temperature sintering ofthe present disclosure allows cold sintering of other substances thatdegrade or oxide at a temperature above 200° C.

Another aspect of the present disclosure includes a process forpreparing a sintered inorganic compound on a substrate. The processincludes depositing an inorganic compound (e.g., a ceramic) on asubstrate (e.g., a substrate comprised of a metal, ceramic, polymer orcombinations thereof). In some embodiments, the inorganic compounds canbe deposited on multiple substrates to form laminates. Solvent can becombined with the inorganic compound before, during or after depositionthereof. In other embodiments, the process includes depositing aninorganic compound (e.g., a ceramic) on a substrate followed combiningthe inorganic compound with a solvent such as by exposing a depositedceramic to an aqueous solvent to form a wetted deposited ceramic. Heatand pressure can be applied to the wetted deposited ceramic to sinterthe ceramic on the substrate. Advantageously, the applied heat can be nomore than 200° C., the applied pressure no more than 5,000 MPa and theceramic can be sintered to a relative density of no less than 85% in ashort period of time.

Embodiments of the present disclosure include one or more of thefollowing features individually or combined. For example, the coldsintering of the present disclosure is applicable to both inorganiccompounds that have congruent dissolution and incongruent dissolution inthe solvent. For inorganic compounds, the solvent can include one ormore source compounds. In some embodiments, the at least one inorganiccompound or ceramic can have a particle size of less than 100 μm, orless than 50 μm, 30 μm, 20 μm, 10 μm, and even less than about 5 μm orless than about 1 μm and into the nanometer regime. In otherembodiments, the solvent can include water with soluble salts and one ormore of a C₁₋₁₂ alcohol, ketone, ester, and/or an organic acid with oneor more soluble salts or source compounds wherein the solvent has aboiling point below about 200° C. In still other embodiments, the heatapplied to the mixture is at a temperature below about 250° C., e.g.,below about 200° C. or below about 150° C., such as below about 100° C.In still further embodiments, the inorganic compound and solvent can becombined by exposing the inorganic compound to a controlled relativeatmosphere of the solvent, e.g., a humid atmosphere when the solvent iswater based, or by mixing the solvent with the inorganic compound suchas mixing a solvent that includes at least 50% by weight of water.

The cold sintering process of the present disclosure can advantageouslyprovide dense sintered materials, e.g., dense inorganic compounds,ceramics, composites. The process of the present disclosure can densifythe material to a relative density of greater than 60%, e.g., greaterthan 80% such as no less than 85% and even greater than 90%. Inaddition, the cold sintering process of the present disclosure candensify the sintered material in short time periods. For example, thecold sintering process of the present disclosure densifies the sinteredmaterial to a relative density of at least 85% and even at least 90% inless than 180 minutes, e.g., less than 120 minutes, such as no more than60 minutes. In some embodiments, the cold sintering process of thepresent disclosure densifies the sintered material to a relative densityof at least 85% and even at least 90% in no more than 30 minutes, forexample.

Additional advantages of the present invention will become readilyapparent to those skilled in this art from the following detaileddescription, wherein only the preferred embodiment of the invention isshown and described, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects, allwithout departing from the invention. Accordingly, the drawings anddescription are to be regarded as illustrative in nature, and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the attached drawings, wherein elements having thesame reference numeral designations represent similar elementsthroughout and wherein:

FIG. 1 illustrates a basic mechanism that can be used for a coldsintering process according to embodiments of the present disclosure.The process is a basic, unique liquid phase sintering process that hasnot been exploited in the manufacture of sintered ceramic materials.

FIGS. 2a-2f show the basic diversity and integration for which coldsintering can be employed.

FIGS. 3a-3e are SEM micrographs of K₂Mo₂O₇ in various forms. The figuresshow one example of microstructural development through varyingprocessing conditions of time, temperature and pressure.

FIGS. 4a to 4c show densification trends for time, temperature andpressure variables. For example, FIGS. 4a to 4c are charts showing therelative densities of K₂Mo₂O₇ ceramics sintered under variousconditions. FIG. 4a is a chart of the relative densities of K₂Mo₂O₇ceramics sintered at 120° C. for 5 min with different pressures; FIG. 4bis a chart showing the relative densities of K₂Mo₂O₇ ceramics sinteredat a pressure of 350 MPa for 5 min with different temperatures; and FIG.4c is a chart showing the relative densities of K₂Mo₂O₇ ceramicssintered at 120° C. at a pressure of 350 MPa with different holdingtimes.

FIGS. 5a-5c are XRD patterns of cold-sintered bulk BaTiO₃ ceramic, andafter annealing at 700-900° C. Impurity phase ˜24° is outlined by thedash circle in (b).

FIG. 5d is a TGA-MS plot for cold-sintered BaTiO₃ ceramic from 30-900°C. Four peaks are marked as P1-P4 on the derivative weight loss curve.

FIG. 5e is a plot of density evolution of cold-sintered and subsequentlyannealed BaTiO₃ ceramics as a function of cold sintering time at 180° C.

FIGS. 6a-6c are plots showing densities of: a) insulator-polymer(LM-PTFE), b) ionic conductor-polymer (LAGP-(PVDF-HFP)), and c)electronic conductor-polymer (V₂O₅-PEDOT:PSS) composites coldco-sintered at 120° C.

FIGS. 7a-7e illustrate electrical and mechanical properties of LM-PTFEcomposites as a function of PTFE volume fraction. FIG. 7a is a plot ofmicrowave permittivity; FIG. 7b is a plot of Q×f values; FIG. 7c is aplot of temperature coefficient of resonant frequency; FIG. 7d is a plotof elastic modulus; and FIG. 7e is a plot of shear modulus.

FIGS. 8a-8d illustrate electrical properties of ionic conductor-polymer(LAGP-(PVDF-HFP)) and electronic conductor-polymer (V₂O₅-PEDOT:PSS)composites. FIG. 8a is a plot of conductivities at 25° C. obtained fromimpedance measurements and FIG. 8b is a plot of activation energies ofLAGP-(PVDF-HFP) composites as a function of PVDF-HFP volume fractionbefore and after soaking in 1 M LiPF₆ EC-DMC (50:50 vol. %). FIG. 8c isa plot of DC conductivities at 25° C. and FIG. 8d is a plot ofactivation energies of V₂O₅-PEDOT:PSS composites as a function ofPEDOT:PSS volume fraction.

FIGS. 9a-b are scanning electron micrographs of a cross-sectional viewof a cold sintered Li₂MoO₄ in a single layered capacitor structure. FIG.9a is a low magnification image of the dielectric cold sintered on PETfilm and FIG. 9b is a high magnification image with top and bottomsilver electrodes.

FIG. 10 is a plot comparing relative density to sintering temperature ofinorganic materials (example BaTiO₃) made by different processes. Thedifferent processes include: Conventional Sintering (CS), Two-StepSintering (TSS), Rate-Controlled Sintering (RCS), Spark Plasma Sintering(SPS), Microwave Sintering (MVS), High Pressure Sintering (HPS), FlashSintering (FS), Combined=RRS (Rapid-Rate Sintering)-RCS-LP (LowPressure)-TSS, and Cold Sintering Processes (CSP) in accordance with thepresent disclosure. A theoretical density of 6.02 g/cm3 is adopted forBaTiO₃. GS=Grain Size.

DETAILED DESCRIPTION OF THE DISCLOSURE

Sintering refers to a process that transforms a solid to a dense solidand typically includes thermal energy and/or pressure. The presentdisclosure relates to cold sintering processes. That is, the presentdisclosure relates to processes to densify materials using a solventthat at least partially dissolves a component of the material and at atemperature within about the boiling point of the solvent andtemperatures up to 200° C. above the boiling point (i.e., coldsintering). Preferably the applied heat is at a temperature at theboiling point of the solvent and temperatures 50 to 80° C. above theboiling point. As used herein the boiling point of the solvent is theboiling point at 1 atmosphere. In some embodiments, the sinteringtemperature is no more than 200° C. The process of the presentdisclosure can achieve dense solids at low temperatures across a widevariety of chemistries and composites.

The process includes combining at least one inorganic compound inparticle form with a solvent that can partially solubilize the inorganiccompound to form a mixture. Other components, e.g., other substances,can also be included with the inorganic compound. The process sinters(e.g., densifies) the inorganic compound, with or without othercomponents, by application of pressure and heat at a temperature toevaporate the solvent. The application of pressure and heat at atemperature to evaporate the solvent advantageously causes the solventto evaporate and densifies the inorganic compound, with or without othercomponents, to form a densified material or composite. The othersubstance is a substance that is different from the at least oneinorganic compound. The other substance can be a different inorganiccompound or it can be a polymer, metal, or other material, for example.

Inorganic compounds useful for the present disclosure include, forexample, ceramics, such as metal oxides, such as lithium metal oxidesand non-lithium metal oxides, metal carbonates, metal sulfates, metalselenides, metal fluorides, metal tellurides, metal arsenide, metalbromides, metal iodides, metal nitrides, metal sulphides, metals andmetal carbides.

We discovered that using fine powders for the at least one inorganiccompound prior to forming the mixture advantageously gave betterproperties for the densified material. Fine powders can be produced bymilling the inorganic compound such as by ball milling, attritionmilling, vibratory milling and jet milling, for example.

In one aspect of the present disclosure, the at least one inorganiccompound in particle form has a particle size of less than 100 μm, orless than 50 μm, 30 μm, 20 μm, 10 μm, and even less than about 5 μm orless than about 1 μm. Particle size can be determined by sedigraphmethods, laser diffraction or equivalent methods in which at least 95%of the particles are at or below the stated particle size.

Advantageously, the temperature applied is no more than about 200° C.above the boiling point of the solvent. It is believed that theapplication of heat causes the solvent to evaporate, supersaturate thesolubilized species and densities the at least one inorganic compound toform the sintered material and/or composite. In an aspect of the presentdisclosure, the heat applied to the mixture is at a temperature belowabout 250° C., e.g., below about 200° C. or below about 150° C., such asbelow about 100° C.

While the pressures applied during the processes of the presentdisclosure are not limited, the materials of the present disclosure canbe sintered under pressure of no more than about 5,000 MPa andpreferably under an intermediate pressure, e.g., about 30 MPa to about2,000 MPa, e.g., from about 250 MPa to about 750 MPa. The pressure canbe applied to aid cold sintering while the solvent can evaporate fromthe system.

Solvents useful in practicing the disclosure include one or more of aC₁₋₁₂ alcohol, ketone, ester, water and/or mixtures thereof. Water canalso be a solvent either alone or with one or more of a C₁₋₁₂ alcohol,ketone, or ester or mixtures thereof with or without a soluble salt.Other components can be added to the solvent to adjust its pH, such asacidic components, including organic acids, e.g., citric acid, aceticacid, formic acid, nitric acid, oleic acid, etc. In an aspect of thepresent disclosure, the solvent can be an aqueous medium including waterwith optionally one or more soluble salts and optionally one or moreC₁₋₁₂ alcohols, ketones, esters, and/or organic acids. Embodimentsinclude an aqueous solvent which includes at least 50% by weight ofwater and one or more other components such an organic acid or one ormore of a C₁₋₁₂ alcohol, ketone, ester, or soluble salt or mixturesthereof. Preferably, the solvent has a boiling point below about 200°C., e.g., below about 120° C.

In one aspect of practicing the present disclosure, water and slightlyacidic water can be added to the material in powder form beforeconsolidation or afterwards in the form of water vapor. Other solventscan be used to control the kinetics of the process, but water workssufficiently well in terms of practice.

The solvent can be combined with the inorganic compound and optionalother components of the mixed with the inorganic compound by directlyadding it to a prepared mixture of a fine powder of the inorganiccompound and optionally other components or by exposing the inorganiccompound and optional other components with vapor from the solvent. Theinorganic compound and optional other components can be under partialpressure during the addition of the solvent. In practicing an embodimentof the present disclosure, the solvent can simply be mixed in a smallamount, e.g., less than about 30% by weight of the total solids such asless than about 0.3 g/lg wt/wt, or by exposing the inorganic compound inpowder form to a controlled relative atmosphere of the solvent such asexposing the inorganic compound to humid atmosphere for an aqueoussolvent.

Advantageously, the cold sintering process densifies the material. Therelative density of the sintered material, e.g., inorganic compound,ceramic or composite, is greater than 60%, e.g., greater than 80% suchas no less than 85% and even greater than 90%. The relative density ofthe sintered material is determined by Mass/Geometry ratio orArchimedes' method or an equivalent method. In addition, the coldsintering process of the present disclosure densifies the sinteredmaterial in short time periods. For example, the cold sintering processof the present disclosure densifies the sintered material to a relativedensity of at least 85% and even at least 90% in less than 180 minutes,e.g., less than 120 minutes, such as no more than 60 minutes. In someembodiments, the cold sintering process of the present disclosuredensifies the sintered material to a relative density of at least 85%and even at least 90% in no more than 30 minutes, for example.

The cold sintering process of the present disclosure is believed to be alow temperature liquid phase sintering process using a solvent, e.g., anaqueous medium, as a transient solvent. For example and in an embodimentof the present disclosure, a ceramic powder is uniformly moisturizedwith a small amount of solvent, e.g., an aqueous solution. It isbelieved that the solid surfaces of the ceramic powder decompose andpartially dissolve in the solvent, so that a controlled amount of liquidphase is intentionally introduced at the particle-particle interfaces.This can be accomplished by simply mixing in a small amount, such as afew drops of the solvent, or exposing the powder to a controlledrelative atmosphere of the solvent such as humid atmosphere for anaqueous solvent. It is believed that the dissolution of sharp edges ofsolid particles of the powder reduces the interfacial areas, and somecapillarity forces aid the rearrangement in the first stage. With theassistance of sufficient external and capillarity pressure, the liquidphase redistributes itself and fills into the pores between theparticles. Applying a uniaxial pressure, the solid particles rearrangerapidly, which collectively leads to an initial densification. Asubsequent growth stage, often referred to as “solution-precipitation”,is created through the evaporation of the solvent that enablessupersaturated state of the liquid phase at a low temperature rightabove the boiling point of the solvent, e.g., right above 100° C. for anaqueous solvent, triggering a large chemical driving force for the solidand liquid phases to reach high levels of densification.

In practicing an embodiment of the present disclosure, ceramics can besintered at low temperature. In such an embodiment, the process includesa ceramic in particle form that is exposed to a solvent, e.g., anaqueous solvent, in an amount of from 1 to 25 wt % whereupon there is apartial dissolution of the ceramic to form a mixture, e.g., a particlebed. This particle bed with solvent can be exposed to a uniaxialpressure and under a controlled drying rate can provide particlerearrangement and precipitation to densify the particles and sinter to adense ceramic, e.g., to a relative density of no less than 85%, such asgreater than 90% in a short time period, e.g., less than 120 minutessuch as 60 minutes or less.

Table 1 below shows materials that have already been demonstrated toundergo cold sintering according to the present disclosure.

TABLE 1 Binary Binary Ternary Ternary Quaternary Quinary CompoundsCompounds Compounds Compounds Compounds Compounds MoO₃ NaCl Li₂CO₃ BiVO₄LiFePO₄ Li_(1.5)Al_(0.5)Ge_(1.5) (PO₄)₃ WO₃ ZnTe CsSO₄ AgVO₃ LiCoPO₄Li_(0.5x)Bi_(1-0.5x)Mo_(x)V_(1-x)O₄ V₂O₃ AgI Li₂MoO₄ Na₂ZrO₃ KH₂PO₄ V₂O₅CuCl Na₂Mo₂O₇ BaTiO₃ Ca₅(PO₄)₃(OH) ZnO ZrF₄ K₂Mo₂O₇ NaNO₂(LiBi)_(0.5)MoO₄ Bi₂O₃ α-Al₂O₃ ZnMoO₄ Mg₂P₂O₇ CsBr ZrO_(2 PSZ) Li₂WO₄BaMoO₄ MgO ZrO_(2 Cubic) Na₂WO₄ Cs₂WO₄ PbTe K₂WO₄ Na_(x)Co₂O₄ Bi₂Te₃Bi₂VO₄ Ca₃Co₄O₉ LiVO₃ KPO₃ SrTiO₃ LiCoO₂

The materials provided in Table 1 were cold sintered in accordance withthe present disclosure to a density of no less than 80% and most of thematerials were cold sintered in accordance with the present disclosureto a density of no less than 85%. PZT materials, such as PbZrTiO₃ canalso be cold sintered.

In bulk form, we have demonstrated wide applications of differentchemistries and crystal structures across binary, ternary, quaternary,quinary oxides, carbonates, fluorides, sulphates, phosphates andbromides that can be cold sintered. The materials selected havepractical interest in the form of dielectric materials, electrochemicalmaterials, ionic electrolytes, mixed ionic conductors, ferroelectrics,semiconducting materials, thermoelectric materials, biomaterials andcatalytic substrate applications. The cold sintering processes disclosedherein can be applicable to arsenides, borides, bromides, carbonates,carbides, fluorides, metals, nitrides, oxides, phosphates, selenides,sulfides, tellurides, etc. with sufficient solubility in the solvent,and kinetics of re-precipitation from the transient supersaturated grainboundary phase is sufficiently fast relative to the heating rates.Sintering of hydroxyapatite (HA) can also be undertaken by the presentcold sintering processes disclosure herein.

In addition to the single phase monolithic substrates that can beproduced under cold sintering, many composites and integrated devicescan be formed through the cold sintering process. These can be in theform of mixed materials, laminated materials, and interconnectedmaterials through thick film methods as used in electronic packages. Thelow temperatures and fast sintering times under cold sintering permitsus to also integrate with the sintered material polymers, nanometals,nanoceramics, biomaterials, biological cells, proteins, drugs, and glassor carbon fibers for mechanical strength, all giving greater designflexibility and functionality that has previously not been available tobulk ceramics and composites. Electroceramics and refractories can alsobe fabricated.

In addition, laminated ceramics can be formed through the tape castingprocess and using low temperature binder systems, such as QPac™,(polyalkylene carbonate) and its appropriate solvents and plasticizers.This can be used to cast the ceramic materials, and then these can belaminated. First, we can remove the binder at temperature 170° C. to200° C. in air or nitrogen atmospheres. These materials can thencarefully be exposed to high humidity to take up water in the surface ofthe particles. After a sufficient time, these unsintered laminates canbe put in a uniaxial press and heated at 100° C. to less than 200° C. asa cold co-sintering process. In other cases, we can just mix volumefractions of polymer powders 0-60% and the ceramics (100% to 40%), pressthem to form the initial shape, then expose the pressed shape tohumidity and then undergo cold sintering to form dense co-sinteredceramics, and ceramic polymer composites. Cold sintering can be done inany gaseous atmosphere, but inert atmospheres are preferred, e.g.,nitrogen or argon, and atmospheres that do not contain measurable CO₂.

FIG. 1 shows basic mechanisms for the cold sintering process accordingto certain embodiments. The process is a basic, unique liquid phasesintering process that has not been exploited in the manufacture ofceramic materials.

FIGS. 2a-2f show the basic diversity and integration contemplated forcold sintering of the present disclosure. Typical examples includemicrowave devices, electronic packages, and thermoelectricenergy-conversion systems, as well as electrochemical systems, such asLi-ion batteries, where polymer separators and binders are interfacedwith ceramic anode and cathode materials. The impact would also be insystems that involve development of nanocomposites, and even simplemonolithic applications, such as substrates, ceramic filters, andcatalytic supports that could be processed at significantly lowertemperatures with fast production times, enabling manufacturing to havehigher throughput, cost- and energy-savings.

The following data are provided to further demonstrate the practice ofthe processes of the present disclosure and characteristics of thesintered materials and composites therefrom.

For example, FIGS. 3a-3e illustrate microstructural development throughvarying processing conditions of time, temperature and pressure. FIGS.3a-3e are SEM micrographs of K₂Mo₂O₇ in various forms. FIG. 3(a) showsthe K₂Mo₂O₇ powder; FIG. 3(b) shows the K₂Mo₂O₇ ceramic sintered at 120°C. for 5 min at a pressure of 350 MPa; FIG. 3(c) shows the K₂Mo₂O₇ceramic sintered at 180° C. for 5 min at a pressure of 350 MPa; FIG.3(d) shows the K₂Mo₂O₇ ceramic sintered at 120° C. for 15 min at apressure of 350 MPa; and FIG. 3(e) shows the K₂Mo₂O₇ ceramic sintered at120° C. for 30 min at a pressure of 350 MPa.

FIGS. 4a to 4c show densification trends for time, temperature andpressure variables. For example, FIGS. 4a to 4c are charts showing therelative densities of K₂Mo₂O₇ ceramics sintered under variousconditions. FIG. 4a is a chart of the relative densities of K₂Mo₂O₇ceramics sintered at 120° C. for 5 min with different pressures FIG. 4bis a chart showing the relative densities of K₂Mo₂O₇ ceramics sinteredat a pressure of 350 MPa for 5 min with different temperatures; and FIG.4c is a chart showing the relative densities of K₂Mo₂O₇ ceramicssintered at 120° C. at a pressure of 350 MPa with different holdingtimes. The data in these figure show how time, temperature and pressurecan affect cold sintering of a give system.

Cold sintering of Li₂MoO₄, Na₂Mo₂O₇, K₂Mo₂O₇, and V₂O₅ illustrate thesintering of sparingly soluble single and mixed metal oxide ceramics. Asshown in FIG. 4a-c , K₂Mo₂O₇ samples are sintered to 89% relativedensity at 120° C. within 5 min under a pressure of 350 MPa (FIG. 4c ).Extending the holding time to 10-20 min, we obtained ceramics withrelative densities >90%, which is comparable to those densities foundwith a conventional thermal sintering temperature of 460° C. Byappropriately varying temperature, pressure, holding time and watercontent, we cold sintered Li₂MoO₄, Na₂Mo₂O₇, K₂Mo₂O₇, and V₂O₅ ceramicsto high density (>90%) phase pure ceramics at a temperature as low as120° C. SEM images indicate that the grain growth is substantiallylimited under these experimental conditions. To this point, the grainsizes of the sintered ceramics could be easily tailored through controlof the initial powder particle size. Such a technique may be utilized toproduce polycrystalline materials with controllable and uniform grainsizes, or even preserve the nanoscale size of the crystallites in thefinal products as shown for LiFePO₄. This set of experimentsdemonstrates the effective use of pressure to enhance the driving forcefor cold sintering. The pressure assists both particle rearrangement andthe dissolution precipitation process at particle contacts.

Since cold sintering involves precipitation of a complex metaloxide-based phase, the appearance of a small fraction of an amorphousphase in the ceramic grain boundaries seems reasonable. The formation ofan amorphous grain boundary phase depends on the rate of solutecondensation as controlled by the rate of solvent evaporation, and theassociated degree of solute super-saturation before condensation of thedissolved phase. We further studied the amorphous-ceramic interface incold-sintered Na₂Mo₂O₇ from the atomic-scale view, with representativecrystallites oriented along their [110] directions.

We observed that the amorphous-crystalline interface is typicallyarranged in a terrace-ledge manner, which is consistent with the classicTerrace-Ledge-Kink (TLK) model used to describe the equilibrium state ofa crystal surface growing from the vapor or liquid; the terrace ends ina ledge and steps down to another one, and the missing atoms in theledge forms kink sites. From a thermodynamic perspective, the stepledges and kinks provide energetically favorable sites for atomicdiffusion and surface free energy minimization during liquid phasesintering, as the ionic species attached to these sites can establish asufficient number of chemical bonds with the crystal surface so as toresist re-dissolving. In the amorphous phase, nanometer-sizedprecipitates are also observed to nucleate on the crystal surface.Additionally, we performed an extensive scanning/transmission electronmicroscopy (S/TEM) study to examine the grain-grain interface region incold-sintered Na₂Mo₂O₇ ceramics. We estimate that 90% of grainboundaries have no amorphous phase, indicating that highly crystallineceramics are approachable.

An advantage of cold sintering processes of the present disclosureincludes the electrical properties of cold sintered Li₂MoO₄, Na₂Mo₂O₇,K₂Mo₂O₇, and V₂O₅ ceramics, which are comparable to those prepared byconventional thermal sintering at 540° C., 575° C., 460° C., and450-660° C., respectively (Table 3, in the Examples section below). Thedata demonstrates that many simple and mixed metal oxides, metalchlorides and composites in a number of crystal structures with avariety of different melting temperatures can be sintered between roomtemperature and 200° C.; certain of the cold sintered inorganiccompounds are listed in Table 1.

The processes of cold sintering of the present disclosure are applicableto both inorganic compounds that have congruent dissolution andincongruent dissolution in the solvent. Congruent dissolution involvessubstantially no change in the composition of the compound upon itsdissolution whereas incongruent dissolution involves a substantialchange in composition of the compound upon its dissolution in thesolvent.

Incongruent dissolution is prevalent in a large number of materials, andwhich also have limited solubility in aqueous media, especially for theclose-packed structures in which the atoms/molecules/ligands aretightened by strong chemical bonding. A well-known example is BaTiO₃,which is not thermodynamically stable in aqueous environment of pH<12.As BaTiO₃ particles react with water, Ba is preferentially leached outfrom the surface area, resulting in a Ba deficient layer with a Ti-richamorphous shell. This amorphous layer is detrimental for theprecipitation process since it separates the solution and crystallattices and significantly impedes crystal growth from thesupersaturated solution by limiting the mass transport between them.Therefore, simply mixing water with a BaTiO₃ powder and applying heatdoes not densify the ceramic.

In an aspect of the present disclosure, inorganic compounds that wouldordinarily incongruently dissolve in a solvent can be cold sintered,with or without other substances. The process includes combining atleast one inorganic compound in particle form with a solvent that canpartially solubilize the inorganic compound to form a mixture. For thisaspect, the solvent is saturated or supersaturated with one or moresource compounds prior to contacting the solvent with the inorganiccompound. The source compounds are preferable compounds that cansynthesize the inorganic compound. Alternatively, the source compoundsare compounds that substantially prevent incongruent dissolution of theinorganic compound when in contact with the solvent. Saturating tosupersaturating the solvent with one or more source compounds prior tocontact with the inorganic compound minimizes or prevents leaching ofelements form the inorganic compound. It is believed leaching is due tothe concentration difference between the solvent and the solid surfaceof the inorganic particles and adding the source compounds to thesolvent to reach the concentration in saturate or supersaturation statesprevents or minimizes leaching. While this aspect of the process of thepresent disclosure is particularly useful for inorganic compounds thatincongruently dissolve in the solvent, it can be used for congruentdissolving compounds as well. The use of solvents including sourcecompounds in the cold sintering process of the present disclosure isdifferent from a process of combining reactive compounds to synthesizean inorganic compound in that the cold sintering process of the presentdisclosure starts with a fully synthesized inorganic compound anddensities the compound rather than to synthesize the compound fromreaction components.

The process continues by applying pressure and heat to the mixture toevaporate the solvent and densify the at least one inorganic compound toform a sintered material or composite. The applied heat in theembodiment is the same as in earlier embodiments, e.g., the applied heatis at a temperature of no more than 200° C. above the boiling point ofthe solvent or at a temperature below about 250° C., e.g., below about200° C. or below about 150° C., such as below about 100° C. The at leastone inorganic compound can include particles sized less than 100 μm, orless than 50 μm, 30 μm, 20 μm, 10 μm, and even less than about 5 μm orless than about 1 μm. A high relative density can be achieved in a shorttime period, e.g., a relative density of at least 85% and even greaterthan 90% can be achieved in less than 180 minutes, e.g., less than 120minutes, such as no more than 60 minutes or no more than 30 minutes.

By practicing a process of cold sintering that includes using a solventwith source compounds, many ceramics that tend to dissolve incongruentlyin aqueous media can be sintered at low temperatures. BaTiO₃ is a goodmaterial to demonstrate the advantages of cold sintering process of thepresent invention because: (1) it is a widely used ceramic material,particularly for multilayer ceramic capacitor (MLCC), (2) a dense BaTiO₃ceramic is generally accomplished at 1200-1400° C. by conventionalthermal sintering, and (3) compared to the micrometer-sized powder,BaTiO₃ nanoparticles are generally more chemically reactive due to theirhigh surface energy. For preparing a cold sintered BaTiO₃, the followingwere employed: (1) high quality BaTiO₃ nanoparticles were employed asthe starting powders; our transmission electron microscopy (TEM) studysuggests that these nanocrystallites are well crystallized withoutnoticeable amorphous phase on their surfaces, and the chemical speciesare uniformly distributed as well; (2) the liquid phase is alwaysmaintained in a supersaturate state with enough amount of Ba source sothat the dissolution of Ba from BaTiO₃ surface is largely inhibited; (3)as with hydrothermal synthesis of BaTiO₃, Ti source is also added to theliquid phase in order to form BaTiO₃, since extensive hydrothermalsynthesis studies have clearly suggest that the formation of BaTiO₃could be achieved at temperatures from room temperature to 300° C. byutilizing simple compounds of Ba and Ti.

FIG. 5a displays the phase structure evolution of as-cold-sinteredBaTiO₃ ceramics and after post annealing at 700-900° C. Further detailswithin a specific angular range are also magnified as FIGS. 5b and 5cfor better illustration. In the as-cold-sintered BaTiO₃ pellet, impurityphase is identified, as circled by the dash line (FIG. 5b ). It has beencommonly reported that BaCO₃ generally appears as a by-product duringhydrothermal synthesis of BaTiO₃ since a certain amount of bariumspecies react with CO₂ at certain temperatures. To this point, it isreasonable to deduce that the impurity phase (˜24°) coincident with the(111) peak of the XRD spectrum of BaCO₃ is most likely owing to theformation of BaCO₃ through the chemical reaction between Ba(OH)₂ and theCO₂ resource in the atmosphere. To improve the phase purity, a postannealing process, as generally reported in the literature, is carriedout at 700-900° C. As expected, the annealing process effectivelyremoves the impurity phase through facilitating the formation of BaTiO₃;all the spectra profiles after annealing perfectly match with theperovskite structure. For the crystal symmetry perspective, the cubicphase seems to maintain unchanged after annealing at the temperatures≤800° C., but an apparent cubic-to-tetragonal phase transformationoccurs after annealing at 900° C., as indicated by the peak splitting˜45°. This crystallographic evolution from cubic to tetragonal symmetryis found to be consistent with the literature.

FIG. 5d illustrates the thermogravimetric property of the cold-sinteredceramic during annealing process. Even though only a slightly totalweight loss of ˜1.8% is observed, sharp changes can still be detected atdifferent temperature stages, and this can be more easily identifiedwhen a weight loss derivative with respective to the temperature isconsidered, as marked by peaks P1-P4. With the assistance of massspectrum, these peaks perfectly correlate with the burning out of twochemical species, the OH⁻ (or H₂O) and CO₂. Firstly, the water vaporcomes off at ˜100° C., which might be attributed to the water detachmentfrom the surface areas of ceramic powders. Heating up to ˜300° C., thedetection of OH⁻ suggests a decomposition of certain hydroxide.Subsequent heating process leads to a consecutive releasing of CO₂,which is primarily observed at two temperature windows centered ˜520° C.and ˜780° C. These results suggest that the chemical reactions almostcomplete at ˜900° C., and the annealing process will be most likely toaffect the density development of the ceramics. To investigate this,FIG. 5e displays the density evolution of cold-sintered BaTiO₃, as wellas corresponding ceramic pellets after annealing at 900° C., as afunction of cold sintering time. Both curves show a similar trend withtwo notable stages: the ceramics cold-sintered less than 30 min exhibitlow density; a boost appears once the sintering time is elongated to 30min, and the density curve keeps an almost plateau configuration afterthat. The density of BaTiO₃ ceramics prepared by cold sintering processcan even reach ˜5.6 g cm⁻³ (˜93% relative dense if the theoreticaldensity of 6.02 g cm⁻³ is adopted) at a surprisingly low temperature(<200° C.) but also in a short time period (about 30 min). These twodensity evolution curves unambiguously indicate that the cold sinteringprocess is determinant to the final density, even though the density canbe slightly improved ˜2% by a post annealing at relatively lowtemperature (700-900° C.) compare to the conventional thermal sinteringtemperature ˜1200-1400° C. for BaTiO₃.

Based on the experimental observations, the underlying mechanism andprimary stages during cold sintering and relative annealing process inBaTiO₃ nanoceramics is believed to occur as follows: BaTiO₃nanoparticles are first homogeneously wetted with the water suspensioncontaining the constituents for hydrothermal synthesis of BaTiO₃. Withthe assistance of external pressure, the liquid phase redistributesitself and fills into the pores between the particles, aiding particlecompaction and rearrangement. Raising up the temperature facilitates thehydrothermal reactions to generate a glass phase, and also speeds up thepartial dissolving of BaTiO₃ surfaces into the solution, resulting in around shape of the crystallite. Once the cold sintering is performed atthe temperature above the boiling point of water, a non-equilibriumdynamic environment is created and preserved until the water content iscompletely consumed. As the water vapor comes off the ceramic ensemble,further compaction proceeds under applied external pressure. As timeelapses, the BaTiO₃ nanoparticles are tightly glued by newly formedglass phase, and a dense (˜93% relative dense compare to BaTiO₃)crystalline and glass phases to reach an equilibrium state;corresponding ionic species and/or atomic clusters (ligands) in theglass phase precipitate on BaTiO₃ crystallites with lower chemicalpotential, as they are thermodynamically more favorable. When theprecipitation process proceeds, the shape of the crystalliteaccommodates: a rounded configuration is generally manifested when theglass phase is prevalent, while polyhedron with flat facets is normallydeveloped when the volume of glass phase is significantly reduced.Simultaneously, mass transport during this process minimizes the excessfree energy of the surface area and removes surface and porosity; theareas of crystallite-crystallite contacts increase, leading to aformation of rigid particulate skeletal network, and also resulting in afurther improvement of the density to ˜95% relative dense.

It has been known that the hydrothermal synthesis of BaTiO₃ is acomplicated process, and the chemical reaction path is highly dependenton the hydrothermal conditions. Even though the mechanism for thehydrothermal synthesis of BaTiO₃ is still under controversy ascontradictive experimental observations have been reported in theliterature, two mechanisms have been primarily proposed: the first oneis the “in-situ transformation (or diffusion reaction) mechanism”, whichassumes that the chemical reaction is initiated at the surface of TiO₂particles and triggers an heterogeneous nucleation process; thedissolved barium diffuses into TiO₂, resulting in a continuous layer ofBaTiO₃ until TiO₂ is completely consumed. The other one is the“dissolution-precipitation mechanism”, which suggests that TiO₂particles first dissolve into the aqueous solution to generate amorphoushydroxytitanium complexes (Ti(OH)^(n−)), and then react with dissolvedbarium to precipitate BaTiO₃ homogeneously from the solution/glassenvironment. In considering of our chemical mapping observations, Tielement is found to be uniformly distributed into the glass phase. Fromthis point of view, it seems to suggest that the presented coldsintering process most likely takes place via thedissolution-precipitation path aided by the epitaxial growth on BaTiO₃particles.

In summary, a dense BaTiO₃ ceramic was successfully obtained atextraordinarily low temperature in contrast to the traditional thermalsintering generally performed at high temperature. Our experiments showthat a highly dense crystal/glass compact (˜93% relative dense compareto BaTiO₃) is firstly obtained at a surprisingly low temperature of 180°C.; then, post heat treatment leads to a thorough crystallization, andfurther improving the density to ˜95% relative dense.

The processes of cold sintering of the present disclosure are applicableto preparing composites of sintered inorganic compounds, e.g. ceramics,with polymers. Co-sintering ceramic and polymers, e.g., thermoplasticpolymers to form composites in a single step with very high volumefractions of ceramics seems unlikely, given the vast differences in thetypical sintering temperatures of ceramics versus polymers. However,these processing limitations can be overcome with the sinteringprocesses of the present disclosure.

In another aspect of the present disclosure, composites including one ormore sintered inorganic compounds with one or more polymers can beformed. The process includes combining at least one inorganic compoundin particle form with at least one polymer and a solvent that canpartially solubilize the inorganic compound to form a mixture.

The process continues by applying pressure and heat to the mixture toevaporate the solvent and densify the at least one inorganic compound toform a sintered material or composite. The applied heat in theembodiment is the same as in earlier embodiments, e.g., the applied heatis at a temperature of no more than 200° C. above the boiling point ofthe solvent or at a temperature below about 250° C., e.g., below about200° C. or below about 150° C., such as below about 100° C. The at leastone inorganic compound can include a certain percentage of particlessized less than 100 μm, or less than 50 μm, 30 μm, 20 μm, 10 μm, andeven less than about 5 μm or less than about 1 μm. A high relativedensity of the inorganic compound can be achieved in a short timeperiod, e.g., a relative density of at least 85% and even greater than90% can be achieved in less than 180 minutes, e.g., less than 120minutes, such as no more than 60 minutes or 30 minutes.

Table 2 below provides an exemplary list of thermoplastic polymerssuitable for cold sintering according to embodiments of the presentdisclosure.

TABLE 2 Common Thermoplastic Polymers Abbreviations Acrylonitrilebutadiene styrene ABS Aliphatic polyamidesPoly(3,4-ethylenedioxythiophene) PEDOT:PSS polystyrene sulfonatePoly(methyl methacrylate) PMMA Poly(p-phenylene oxide) PPEPolybenzimidazole PBI Polycarbonate PC Polyetherether ketone PEEKPolyetherimide PEI Polyethersulfone PES Polyethylene PE Polypropylene PPPolystyrene PS Polytetrafluoroethylene PTFE Polyurethanes Polyvinylchloride PVC Polyvinylidene difluoride PVDF Sulfonatedtetrafluoroethylene (Nafion)

The sintering conditions of the present disclosure make it possible toco-sinter polymers and ceramic materials in a one-step sinteringprocess. Three illustrative examples include: microwave dielectricLi₂MoO₄—(—C₂F₄—)_(n) (PTFE), electrolyteLi_(1.5)Al_(0.5)Ge_(1.5)(PO₄)₃—(—CH₂CF₂—)_(x)[—CF₂CF(CF₃)—]_(y)(PVDF-HFP), and semiconductor V₂O₅-PEDOT:PSS(poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) composites toshow a range of electrical functionalities created by the ability toco-process dense ceramics with polymers as the minor filler phase intopreviously unrealized composites. We select these composites todemonstrate new designs for dielectric property, electricalconductivity, and both electronic and ionic conductivity. Given theguidance and data in the present disclosure, polymer manufacturingapproaches can be modified for the sintering of both ceramics andceramic-polymer composites, resulting in saving very large amounts ofenergy in production, increasing throughput, and also allowing novelcomposite designs.

Dense Li₂MoO₄ (LM), Li_(1.5)Al_(0.5)Ge_(1.5)(PO₄)₃ (LAGP) and V₂O₅ceramics can be cold sintered at 120° C. for 15-60 minutes, as shown inFIG. 6a-c , in contrast to high conventional thermal sinteringtemperatures and long holding times at 540° C. for 2 hours, 825° C. for8 hours, and 450-660° C. for 2 to 26 hours, respectively. The polymer isa light weight material, and thereby the densities of theceramic-polymer composites decrease with increasing amount of polymer(FIG. 6a-c ). The polymers were specifically selected to compliment theproperties of the ceramic materials. For example, PTFE is a very gooddielectric material, PVDF-HFP is an excellent host for Li-salts inpolymer gel electrolytes, and PEDOT:PSS is a good electronic conductingpolymer. The relative densities of all the (1-x) LM-x PTFE and (1-x)V₂O₅-x PEDOT:PSS samples are higher than 90%, and the densities of (1-x)LAGP-x (PVDF-HFP) samples range between 80 to 88%, indicating that theceramic-polymer composites can be sintered well by a cold sinteredprocess. In contrast to the fluoropolymers PTFE and PVDF-HFP, thePEDOT:PSS is a hydrophilic polymer with a pH value around 1.5-2.5 inaqueous solution (3-4% PEDOT:PSS). The lower pH value of PEDOT:PSS canenhance the dissolution rate of V₂O₅ in water and slightly improve therelative density of (1-x) V₂O₅-x PEDOT:PSS composites.

There are no obvious impurity phases after the cold sintering, revealingthat the ceramics and polymers can be co-sintered, and two-phasecomposites were formed. Using cold sintering according to the presentdisclosure allows preparation of dense samples with small and largeamounts of polymer. When a small amount of polymer is used in thecomposition, the ceramic acts as the matrix material and the polymer thefiller and when a large amount of polymer is used in the composition,the polymer acts as the matrix material and the ceramic the filler. Backscattered images and energy dispersive spectroscopy (EDS) maps ofceramic-polymer composites indicate that the densities of (1-x) LM-xPTFE, (1-x) LAGP-x (PVDF-HFP) and (1-x) V₂O₅-x PEDOT:PSS samples arerelatively high, which is consistent with the density results above. Itis also observed that good dispersion composites can be obtained aftercold sintering at 120° C.

The performance of ceramic-polymer composites depends on the propertiesof the component materials, their volume fractions, phase connectivity,particle sizes, porosity, etc. Dense ceramic-polymer composites can beobtained by cold sintering according to the present disclosure.Therefore, by changing the amount of polymer, it is possible to designthe properties of ceramic-polymer composites, such as electrical andmechanical properties, as illustrated in FIG. 7a . The microwavedielectric (electrical) properties of (1-x) LM-x PTFE composites as afunction of x value are plotted in FIG. 7b-d . The permittivity of PTFEis lower than that of Li₂MoO₄, so that the relative permittivity ofcomposites decrease from 5.8 to 2.9, with x increasing from 0 to 0.7. Anumber of models have been proposed to predict the average permittivityof two-phase or multi-phase composites. The simplest model is assumingthat the composite materials are aligned parallel and perpendicular(series) to the electric field, which derives the upper bound(Equation 1) and lower bound (Equation 2) of the relative permittivityof the composite, ε, respectively:

ε=V ₁ε₁ +V ₂ε₂  (1)

1/ε=V ₁/ε₁ +V ₂/ε₂  (2).

Where ε₁ and ε₂ are relative permittivities of phase 1 and phase 2,respectively; V₁ and V₂ (V₁+V₂=1) are the volume fractions of these twophases. As shown in FIG. 7b , the measured relative permittivity of(1-x) LM-x PTFE composites are lower than the calculated ones obtainedfrom parallel mixing law and higher than that calculated from seriesmixing law. Generally, the assumption of either perfectly parallel orperpendicular alignment is not appropriate for the real samples, andmany modified models are deduced. Considered as a probability problem,the relative permittivity of the composite can be derived from theprinciples of statistics:

ε^(n) =V ₁ε₁ ^(n) +V ₂ε₂ ^(n)(−1≤n≤1)  (3)

When n=1 and n=−1, Equation 3 becomes parallel and series mixing laws,respectively. For a random distribution system, n approaches 0 andEquation 3 gives the expression of logarithmic mixing law:

ε=ε₁ ^(V) ¹ ε₂ ^(V) ² i.e. lgε=V ₁ lgε ₁ +V ₂ lgε ₂  (4)

FIG. 7a shows that the measured permittivity data are in good agreementwith the trends predicted by Equation 4.

Quality factor (Q), the reciprocal of loss tangent (Q=1/tan δ) is animportant parameter to denote the energy loss of the microwave system.FIG. 7c shows that the Q×f (f, resonant frequency) value has no obviousdeterioration when the amount of PTFE changes, indicating that the (1-x)LM-x PTFE composite can be used for microwave application. We alsodemonstrate that the density, permittivity and Q×f value of (1-x) LM-xPTFE composites with a high volume fraction of ceramic can be improvedby cold sintering, in contrast to conventional hot press process, asshown in Table 4 (provided in the Examples section below). Temperaturecoefficient of resonant frequency (TCF) represents the thermal stabilityof materials and can be obtained from the slope of thetemperature-resonate frequency curve, TCF=1/f₀·df/dT. LM and PTFE havedifferent TCF values, therefore, with x increasing from 0 to 0.7, theTCF value of (1-x) LM-x PTFE composites shift from −170 to −7.2 ppm °C.⁻¹ (FIG. 7d ). This result reveals that the thermal stability ofresonate frequency of LM can be improved by adding PTFE. A simpleassumption to predict the TCF values of composites is the linear mixingrule, which is derived from the logarithmic mixing law of permittivity:

TCF=V ₁TCF₁ +V ₂TCF₂  (5)

Where TCF₁ and TCF₂ are TCF values of phase 1 and phase 2, respectively.It is seen that the experimental TCF values are similar to thepredictions of Equation 5.

Polymers are relatively soft materials compared to ceramics which arestiff materials, so that the elastic and shear moduli of the (1-x) LM-xPTFE composites decrease with increasing PTFE content, as shown in FIG.7e . Similar to the prediction of permittivity, there are numerousmodels to calculate elastic/shear modulus of composites. The upper andlower bounds can be determined assuming that the composite materials arealigned parallel and perpendicular (series) to the direction of loading,respectively. Generally, the modulus lies between the upper and lowerbounds, as demonstrated in FIG. 7e . Here again using the logarithmicmixing rule, the measured modulus of composites has good agreement withthe calculated one. When the amount of PTFE 15 large, the measuredmodulus is a little smaller than that of calculated one. In this region,PTFE can be considered as the matrix and the ceramic is the filler. Manyother models can be used to predict the modulus of (1-x) LM-x PTFEcomposites.

Amorphous regions of the PVDF-HFP copolymer absorb liquid electrolytewhen soaked. Therefore, composite electrolytes were soaked in liquidelectrolyte to boost ionic conductivity. Conductivities at 25° C. of(1-x) LAGP-x (PVDF-HFP) composites soaked in 1 M LiPF₆ EC-DMC (50:50vol. %) ranged from 3.3×10⁻⁵ to 1.4×10⁻⁴ S cm⁻¹, while activationenergies ranged from 0.28 to 0.43 eV (FIGS. 8a and 8b ).Well-crystallized, conventionally sintered LAGP has a conductivity of3×10⁻⁴ S cm⁻¹ at 25° C. At temperatures <0° C., the liquid electrolytefreezes out, while >50° C., the liquid electrolyte dries out. Therefore,outside of the modest temperature range of 0° C. to 50° C., conductivityis not stable with holding time.

The total activation energy of cold sintered LAGP with and withoutpolymer is consistent with a partially amorphous grain boundary (0.60eV). Grain boundaries dominate the total conduction and total activationenergy of cold sintered LAGP with and without polymer. In contrast,well-crystallized, conventionally sintered LAGP grain and grain boundaryregions have similar activation energies (0.40±0.02 eV). Literaturedescribes the origin of grain boundary resistance to be geometricalcurrent constriction from limited grain boundary contact area. Whileco-sintering ceramic with polymer may physically bridge resistive grainboundaries, soaking the composite in liquid electrolyte is required toionically bridge these resistive grain boundaries. Polymer swellingthrough liquid electrolyte uptake also increases grain boundary contactarea. Compositions with polymer loadings ≥30 vol. %, where polymerswelling changes the composite dimensions, has been reported inflexible, solvent cast composites. After sixty days of soaking in liquidelectrolyte at room temperature, dimensions of composite electrolytes ofx≤0.30 in (1-x) LAGP-x (PVDF-HFP) did not change. No change in compositeelectrolyte dimensions is related to the cold sintered ceramicconstraining the polymer's swelling.

V₂O₅ is a wide bandgap semiconductor, which has an electronic DCconductivity (σ) of 10⁻⁵˜10⁻³ S cm⁻¹ at room temperature and activationenergy of 0.17-0.21 eV.^([12]) The DC conductivity (4.8×10⁻⁴ S cm⁻¹) andactivation energy (0.25 eV) of cold sintered V₂O₅ ceramics arecomparable to that obtained by the conventional thermal sintering.Through the addition of PEDOT:PSS with a high conductivity of 150 S cm⁻¹(18 μm film) at room temperature, further enhanced conductivity can beconsidered. This is demonstrated in cold sintered composites with the DCconductivity improving systematically with PEDOT:PSS additions, as shownin FIG. 8c . Surprisingly, the DC conductivity of (1-x) V₂O₅-x PEDOT:PSScomposites can be increased by 1-2 orders only by adding up to 1-2%PEDOT:PSS. The activation energy of (1-x) V₂O₅-x PEDOT:PSS (0.8≤x≤0.27)composites is in the range of 0.22-0.23 eV, and lower than that of pureV₂O₅ ceramic (FIG. 8d ).

In summary, ceramic-polymer composites can be prepared using coldsintering processes of the present disclosure. The composites caninclude a wide range of polymers and can be sintered to high densitiesby a low sintering temperature (e.g., as low as 120° C.) and for shorttime periods (e.g., ranging from 15-60 minutes). The electrical andmechanical properties of composites can be predicted by the mixing law.The cold sintering processes of the present disclosure can bridge theprocessing gap of ceramics and polymers, and open up a simple andeffective way for material systems and devices using ceramics andpolymers that are traditionally incompatible. Typically, hundreds ofdegrees separate the ability to co-process these materials in one stepwith high volume fractions of ceramic materials. Hence, the coldsintering processes of the present disclosure allows fabrication ofsintered materials and composites that include a substance that degradesor oxides at a temperature above about 200° C.

Cold sintering allows for the fabrication of new materials and devicesdue primarily to its the low temperature process. For example, the coldsintering process of the present disclosure allows densification ofdifferent materials such as ceramics, polymers and metals on the samesubstrate to obtain functional circuitry. Such materials and devices canbe fabricated by depositing a ceramic, such as a ceramic paste, with orwithout other substances, on to a substrate (e.g., a substrate comprisedof a metal, ceramic, polymer). The substrate can have an electrode layerbetween the deposited ceramic and substrate among other device layers.After deposition, the ceraminc can be combined with a solvent such as byexposing the deposited ceramic to an aqueous solvent to form a more orless uniformly wetted deposited ceramic. Heat and pressure can then beapplied to the deposited and wetted ceramic to sinter the ceramic on thesubstrate in the same manner as heat and pressure were described forother embodiments. In an embodiment, the process can be heated to lessthan 200° C., e.g., less than 150° C., with a pressure of no more than5,000 MPa, e.g., less than 2,000 Mpa, or between about 30 Mpa to about1,000 MPa. By the cold sintered process, the sintered ceramic on thesubstrate can achieve a relative density of greater than 80% such as noless than 85% and even greater than 90%. In addition, the high relativedensity of the ceramic can be achieved in a short time period, e.g., arelative density of at least 85% and even greater than 90% can beachieved in less than 180 minutes, e.g., less than 120 minutes, such asno more than 60 minutes and even no more than 30 minutes. Fabrication ofcold sintered capacitors on both metal and polymeric substrates areprovided in the examples below.

EXAMPLES

The following examples are intended to further illustrate certainpreferred embodiments of the invention and are not limiting in nature.Those skilled in the art will recognize, or be able to ascertain, usingno more than routine experimentation, numerous equivalents to thespecific substances and procedures described herein.

Powder Preparation. NaCl (99.9%), Li₂CO₃ (99%), Na₂CO₃ (99.95%), K₂CO₃(99%), MoO₃ (99.5%), WO₃ (99.8%), V₂O₃ (99.7%), CsBr (99%), CsSO₄ (99%),ZnMoO₄ (98%), Gd₂O₃ (99.99%), Na₂WO₄-2H₂O (95%), LiVO₃ (99.9%), BiVO₄(99.9%), AgVO₃, Na₂ZrO₃ (98%), KH₂PO₄ (99%), and citric acid monohydrate(99.5%) were purchased from Alfa Aesar. BaFe₁₂O₁₉ (97%) nanopowders,V₂O₅ (98%), and ZnO (99%) were provided by Sigma-Aldrich. Bi₂O₃ (99.9%),E-glass fibers (70 μm) and Teflon (PTFE) were purchased from MCP Inc.,TAP Plastics Inc. and Howard Piano Industries, respectively. Li₂MoO₄,Na₂Mo₂O₇, K₂Mo₂O₇ and Li₂WO₄ powders were synthesized by the solid statereaction method using stoichiometric amounts of Li₂CO₃, Na₂CO₃, K₂CO₃,MoO₃ and WO₃. Mixtures of raw materials were milled with zirconia ballsin ethanol for 24 h. After being dried, the powders were calcined in airat 450-600° C., followed by a second ball mill with ZrO₂ and ethanol for24 h. Then, some Li₂MoO₄ powders were mixed with BaFe₁₂O₁₉, PTFE, andE-glass fibers (abbreviated as “EG”) according to the following formula:0.8Li₂MoO₄-0.2BaFe₁₂O₁₉, 0.5Li₂MoO₄-0.5PTFE, and 0.8Li₂MoO₄-0.2EG(Volume Fraction). The mixtures were ball milled in ethanol and thendried. The particle sizes for these materials were accessed with SEMScanning electron microscopy and the particle sizes were in the range of0.5 to 10 microns.

Bulk Ceramic Preparation. Dense ceramics of MoO₃, WO₃, V₂O₃, V₂O₅, ZnO,Bi₂O₃, CsBr, Li₂CO₃, CsSO₄, Li₂MoO₄, Na₂Mo₂O₇, K₂Mo₂O₇, ZnMoO₄,Gd₂(MoO₄)₃, Li₂WO₄, Na₂WO₄, LiVO₃, BiVO₄, AgVO₃, Na₂ZrO₃, LiFePO₄, andKH₂PO₄, and dense composites of Li₂MoO₄-BaFe₁₂O₁₉, Li₂MoO₄-PTFE, andLi₂MoO₄-EG were prepared via the following methods including acold-sintering step.

Method 1: All the powders except ZnO were mixed with 4˜25 wt % deionizedwater using a pipet. ZnO was mixed with an aqueous acetic acid solventwith concentration of 0.5-5.0 M acetic acid (pH value of about 2-4).After being stirred with a mortar and pestle, the moistened powders werehot-pressed with a steel die into dense pellets (12.7 mm in diameter and1-5 mm in height) under a uniaxial pressure of 80˜570 MPa at 120° C. Thedie was preheated at 120° C. for more than 1 h. Finally, the pelletswere placed into an oven at 120° C. for 6 h to remove the possibility ofwater residue.

Method 2: All the dry powders were pressed into soft pellets under a lowpressure (30˜70 mPa) at room temperature. Then, the pellets were put ina humid atmosphere (water vapor generated by heating deionized water orhumidity chamber) for 10-360 min. The moistened pellets were hot-pressedwith a steel die into dense pellets under a uniaxial pressure of 80˜570MPa at 120° C. The die was preheated at 120° C. for more than 1 h.Finally, the pellets were placed into an oven at 120° C. for 6 h toremove the possibility of water residue.

Multilayer Ceramic Preparation. Li₂MoO₄ and K₂Mo₂O₇ tapes were preparedby the tape casting procedure. The powders were first added into asolution of 96 wt % methylethylketone (MEK) and 4 wt % Qpac, and milledwith zirconia balls. Then, another solution of 66.3 wt %methylethylketone (MEK), 28.4 wt % Qpac and 5.3 wt % Santicizer-160 wasadded into the slurry, followed by an additional ball milling. Tapecasting was performed using a laboratory-type casting machine with adoctor blade casting head. Silicone-coated mylar (polyethyleneterephthalate) was used as a carrier film. The cast slurry was dried atroom temperature. For Li₂MoO₄—K₂Mo₂O₇ multilayer, Li₂MoO₄ and K₂Mo₂O₇green tapes were stacked alternately. For Li₂MoO₄—Ag multilayer, silverpaste was printed on the Li₂MoO₄ green tape and two silver-printedlayers and ten Li₂MoO₄ layers were stacked together. Then, the stackedLi₂MoO₄—K₂Mo₂O₇ and Li₂MoO₄—Ag layers were laminated under an isostaticpressure of 20 MPa at 80° C. for 20 min. The binders were burn out at175° C. for 10 h in air with a heating rate of 0.5° C./min. Themultilayers were sintered using a cold-sintering fabrication step, asdescribed previously. In particular, the multilayers were put in a humidatmosphere (water vapor generated by heating deionized water or humiditychamber) for 10-360 min. Then, the moistened multilayers werehot-pressed with a steel die into dense ceramics under a uniaxialpressure of 80˜570 MPa at 120° C. The die was preheated at 120° C. formore than 1 h. After cold sintering, the co-fired multilayers wereplaced into an oven at 120° C. for 6 h to remove the possibility ofresidual hydroxides.

The bulk densities of the sintered samples were measured byMass/Geometry ratio and Archimedes' method. Relative densities weredetermined by the ratio of experiment measured bulk density over thedensity of corresponding density of the materials in the form of singlecrystals.

Table 3 below provides densities and performance characteristics forcertain ceramics prepared with a cold sintering step including water asa solvent and at 120° C. under a pressure of 350 MPa.

TABLE 3 Cold Sintering Process Conventional (CSP) Thermal DensityRelative Sintering Process (g/cm³) Density Performance PerformanceLi₂MoO₄ 2.9 95.7% ε_(r) = 5.6 ε_(r) = 5.5 Q × f = 30,600 GHz Q × f =46,000 tanδ = 5.7 × 10⁻⁴ GHz (17.4 GHz) Na₂Mo₂O₇ 3.45 93.7% ε_(r) = 13.4ε_(r) = 12.9 Q × f = 14,900 GHz Q × f = 62,400 tanδ = 7.5 × 10⁻⁴ GHz(11.2 GHz) K₂Mo₂O₇ 3.39 94.1% ε_(r) = 9.8 ε_(r) = 7.5 Q × f = 16,000 GHzQ × f = 22,000 tanδ = 8.3 × 10⁻⁴ GHz (13.3 GHz) V₂O₅ 3.03 90.2% σ_(c) =4.8 × 10⁻⁴ S/cm σ_(c) = 10⁻⁵~10⁻³ S/cm ε_(r), microwave permittivity.tanδ, loss. Q, quality factor (Q = 1/tanδ). f, resonate frequency.σ_(c), DC conductivity.

Hydrothermal Assisted Ceramic Processing. BaTiO₃ nanopowders (99.9%, 50nm with cubic phase) were purchased from commercial resource (e.g., USResearch Nanomaterials, Inc.). Ba(OH)₂/TiO₂ suspension was made bymixing corresponding chemicals with deionized water. The molar ratio ofBa(OH)₂:TiO₂ was 1.2:1, and the concentration of Ba(OH)₂ was 0.1 molL⁻¹. To form a ceramic pellet, 0.14-0.15 g Ba(OH)₂/TiO₂ suspension wasadded to 0.56 g BaTiO₃ nanopowders; the mixtures were grinded usingpestle and mortar for 3 minutes. The mixture was uniaxially pressedunder 430 MPa first at room temperature (25° C.) for 10 minutes, andthen the temperature was ramped up to 180° C. with an overall rate of 9°C. min⁻¹. The temperature was isothermally kept for 1 minute to 3 hoursto obtain a series of samples. The as-prepared ceramic pellets werefirst baked at 200° C. overnight to further remove possible waterresidue, and then further annealed at 700-900° C. for 3 hours with atemperature ramp rate of 5° C. min⁻¹ in air. The densities were measuredby Archimedes method using acetone (0.791 g cm⁻³) as a liquid media.

The phase structures were checked by X-ray diffraction (Panalytical,X'Pert PRO) with Cu-Kα radiation. For dielectric measurements, platinumwas sputtered as electrodes, and the dielectric properties were measuredat 1 kHz-1 MHz by LCR meter (HP4284A, Agilent Technologies) duringcooling from 200° C. to room temperature with 2° C. min-1 rate.Thermogravimetric-Mass Spectrum (TGA-MS Q50, TA Instrument) analysis wasperformed in helium atmosphere from 30 to 900° C. with 10° C. min-1.Ceramic powders crushed from the sintered pellets were used. Beforeheating up, the samples were kept at 30° C. for 1 hour to reach anequilibrium state. TEM specimens were prepared via standard proceduresincluding mechanical thinning, polishing, and ion milling. The specimenswere polished down to ˜30 μm thick, and then mounted on molybdenumgrids. The foils were further thinned with an Ar-ion mill (Gatan, PIPSII) until an electron transparent perforation was formed. A cryogenicstage was used to cool the specimen to the liquid N₂ temperature duringion milling to minimize structural damage and artifacts. Microstructuraland chemical studies were performed on a Talos (FEI, Talos) microscopyequipped with an Energy Dispersive X-ray Spectroscopy (EDS) systemoperating at an accelerating voltage of 200 kV.

Ceramic/Polymer Composites.

(1-x) LM-x PTFE powder preparation: To obtain fine powders, Li₂MoO₄(Alfa Aesar, 99%) was ground with a mortar and then ball milled inethanol for 48 h. After being dried, the Li₂MoO₄ powder was mixed withPTFE (Howard Piano Industries) according to the following composition:(1-x) LM-x PTFE (x=0, 10, 40, 50, 60, 70 vol. %). The mixture was ballmilled in ethanol for 24 h, followed by drying at 85° C.

Li_(1.5)Al_(0.5)Ge_(1.5)(PO₄)₃ powder preparation: Stoichiometricamounts of Li₂CO₃ (Alfa Aesar, 99%), Al₂O₃(Tape Casting Warehouse,Inc.), GeO₂ (Alfa Aesar, 99.98%), and NH₄H₂PO₄ (Alfa Aesar, 98%) wereball milled for 24 h, calcined in air at 750° C. for 30 min, and againball milled for 24 h. Milled powder was placed in a covered aluminacrucible and melted in air at 1380° C. for 1 h before beingsplat-quenched. Splat-quenched glass was annealed at 450° C. for 3.75 hand crystallized in air at 825° C. for 8 h. Glass-ceramic powder wassieved through a 74 μm mesh.

(1-x)V₂O₅-x PEDOT:PSS powder preparation: V₂O₅ powders (Sigma Aldrich,98%) were first dispersed in deionized water and then mixed withPEDOT:PSS solution (Sigma Aldrich, High-Conductivity Grade, 3-4% in H₂O)in proportion (x=0, 0.8, 1.6, 3.2, 14.9, 27 vol. %). The mixture wasstirred at room temperature for 4 h, and dried at 120° C.

Composite Cold Sintering: Dense composites were prepared by a coldsintering process. In the case of (1-x) LM-x PTFE, appropriate amountsof deionized water (6 to 12 wt. %) were added to the mixture of Li₂MoO₄and PTFE, and mixed homogeneously with a mortar and pestle. Afterwards,the moistened powders were pressed into dense samples under 350 MPauniaxial pressure at 120° C. for 15-20 min. In the case of (1-x) LAGP-x(PVDF-HFP), 30 to 39 vol. % of deionized water was added to LAGP andhomogenized in a mortar and pestle. LAGP and PVDF-HFP (Arkema, KynarPowerflex LBG) were mixed by swirling in liquid nitrogen and pressedunder 400 MPa uniaxial pressure at 120° C. for 1 h. For (1-x) V₂O₅-xPEDOT:PSS, moistening took place with deionized water (11 to 17 wt. %)and dense ceramic-polymer composites were pressed under a uniaxialpressure of 350 MPa at 120° C. for 20-30 min.

Characterization: The phase purity and composition of composites aftercold sintering were determined by X-ray diffraction (PANalyticalEmpyrean). An environmental scanning electron microscope (ESEM, FEI,Quanta 200) with an energy dispersive spectrometer (EDS) was used toanalyze microstructures and elemental distribution of Cold Sinteredsamples on cross section. Bulk densities were measured by mass/geometricvolume ratio and Archimedes method. The microwave dielectric properties(permittivity and Q×f valve) of (1-x) LM-x PTFE were identified usingthe Hakki-Coleman method in TE₀₁₁ mode with a vector network analyzer(Anritsu 37369D). The temperature coefficient of resonant frequency(TCF) value was obtained using TE_(01δ) shielded cavity method with anetwork analyzer, cylindrical resonant cavity and temperature chamber(Delta 9023, Delta Design, Poway, Calif.), and calculated as follows:TCF=(f₈₅−f₂₅)/(f₂₅(85−25))·10⁶ ppm ° C.⁻¹, where f₂₅ and f₈₅ were theresonant frequencies at 25° C. and 85° C., respectively. The elastic andshear modulus measurements were performed by sound velocity method basedon ASTM standard E 494-05. To perform impedance measurements on Li-ionconducting (1-x) LAGP-x (PVDF-HFP) composites, Au electrodes, 100 nmthick, were sputtered on the pellet faces. Pellets were soaked in 1 MLiPF₆ EC-DMC (50:50 vol. %) (BASF Selectilyte LP 30) at 25° C. inside anAr glovebox and wiped of excess liquid before being loaded intoair-tight cells for impedance spectroscopy (Solartron Ametek ModuLab).Uptake of liquid was 5 to 10 wt. % (10 to 12 μL). In order to measurethe DC electrical conductivity on (1-x) V₂O₅-x PEDOT:PSS, the disks werecut into bars with a dimension of 10×2×1 mm. After polishing the bars,Pt electrodes were deposited and Ag wires were attached with Ag epoxy(Epo-tek H20E). The DC electrical conductivity was measured using a fourterminal technique with a Keithley 2700 Integra series digitalmultimeter.

Table 4 below provides densities and dielectric properties of0.9Li₂MoO₄-0.1PTFE composite prepared by Hot Press and Cold SinteringProcess (CSP)

TABLE 4 Density Relative Q × f (g cm⁻³) Density (%) ε_(r) (GHz) tanδ HotPress 2.5 85 4.7 10430 1.7 × 10⁻³ (22.9 GHz) CSP 2.85 95.8 5.2 25150 7.2× 10⁻⁴ (18.1 GHz)

Electroceramics

A ceramic ink was prepared using Lithium Molybdenum Oxide powder (99+%,Alfa Aesar, Ward Hill, Mass.) that was ball-milled in ethanol for 48-100hours before use. A printing vehicle was made by mixing QPAC 40(poly(propylene carbonate)) resin (Empower Materials, New Castle, Del.)with Ethylene Glycol Diacetate (97%, Alfa Aesar, Ward Hill, Mass.) inamounts of 15 and 85 wt % respectively in a planetary centrifugal mixer(AR250, Thinky USA, Laguna Hills, Calif.) until the resin was completelydissolved in the solvent. To formulate the ink, LizMoat, the printingvehicle, additional ethylene glycol diacetate and Butyl Benzyl PhthalateS-160 (Tape Casting Warehouse, Morrisville, Pa.) in amounts of 66.1,22.0, 11.0, and 0.9 wt % respectively were blended and homogenized inthe planetary centrifugal mixer.

Substrates were prepared by cutting PET (Polyethylene terephthalate)sheets (Tape Casting Warehouse, Morrisville, N.J.) into 32 by 32 mmsquares, and then metallized using silver ink (5029, DuPont, Wilmington,Del., USA) in a 25.4 by 25.4 mm square pattern to form a bottomelectrode. A 200 mesh stainless steel screen (UTZ LLC, Little Falls,N.J.) was used to print the pattern. The silver ink was cured at 120° C.for 30 minutes. Alternatively, 50 micron thick Nickel foil (99+%, AlfaAesar, Ward Hill, Mass.) substrates were also prepared. A ceramic inkwas printed onto the metallized PET substrates using a 400 calendaredmesh stainless steel screen with a pattern of 5 by 5 mm squares. Adouble pass was used for each printed layer of the double-layeredprints, where the ink was dried at 120° C. for 15 minutes betweenlayers. Ceramic ink was also printed onto the Nickel Foil substratesusing a 25.4 by 25.4 mm square pattern, 200 mesh screen as describedabove. Single layered prints were dried as above. Binder burnout wasperformed at 0.2° C./min to 150° C. for the Nickel samples, and 175° C.for the PET samples, with a dwell at peak temperature for 6 hours.

Cold Sintering was performed by first exposing the printed samples towater vapor in a sealed beaker at 35-40° C. until the bright whiteprints just turned a dull gray color, which indicated that water hadabsorbed into the printed ink squares. Wetted samples were allowed tosit for approximately a minute to allow excess water to equilibratebefore placing a silicone coated PET sheet on top of the printed film.The sample was then placed between several sheets of paper orPolytetrafluoroethylene (PTFE) sheets and loaded into the platen presspreheated to 120° C. The paper and PTFE were used to help evenlydistribute pressure on the samples with a 70 to 100 MPa pressure beingapplied to the stacks for 12-15 minutes. At the end of the laminationcycle, the paper or PTFE and PET film was carefully removed from thesample, and the printed film displayed a translucent light gray color.Top electrodes were applied in a circular configuration using colloidalsilver paste (PELCO, Ted Pella, Redding, Calif.). The electrodes wereallowed to dry at room temperature for 10 minutes.

Capacitance, C and loss (tan δ) at room temperature, 1 kHz were measuredusing an LCR meter (Model E4980, Agilent, Santa Clara, Calif.) for thePET samples and for the Nickel foil samples, an LCR meter (Model SR715,Stanford Research, Sunnyvale, Calif.) was used. Thickness, t, wasmeasured by using a dial gauge (Model ND280, Heidenhain, Traunreut,Germany). The relative permittivity, ε_(r) was calculated from aparallel plate capacitor approximation, using the area, A, of the topelectrode, and the formula, C=ε₀ε_(r)A/t. The microstructure of theprinted samples was studied by using an environmental scanning emissionmicroscope (E-SEM, FEI Quanta 200, Hillsboro, Oreg.).

Capacitance and loss at room temperature, 1 kHz were measured andpermittivity was calculated based on a determined print thickness of20-21 microns for the single-layered capacitors on Nickel foil, and30-31 microns for the double-layered printed capacitors on PET film. Itappears that lamination conditions, such as the type of paper or plasticsurrounding the sample during pressing can influence electricalproperties. It was observed that several sheets of glossy paper gave thesmoothest, and most even lamination condition, as all 9 capacitors inthe array appeared to be densified. This was indicated by a color changefrom white, which represented green state samples to a dull gray whichindicated densified samples, and similar electrical characteristics.Results of the averaged dielectric properties are summarized in Table 5along with various modifications to the process. The reported dielectricproperties for Li₂MoO₄ fired at 540° C. and measured at room temperatureat 13.051 GHz is a relative permittivity of 5.5. See Zhou et al.,“Microwave Dielectric Ceramics in Li₂O—Bi₂O₃—MoO₃ System with Ultra-LowSintering Temperatures,” J. Am. Ceram. Soc., 93 [4] 1096-100 (2010).FIGS. 9a-9b shows the cross-sectional view of printed and sinteredLi₂MoO₄ ceramic ink with Ag electrodes on PET film. The Li₂MoO₄ ceramiccoexists with Ag electrodes, indicating that the printed Li₂MoO₄ ceramiccan be co-sintered with Ag electrodes by a cold sintering process at120° C.

TABLE 5 Summary of Electrical Properties of Cold sintered Li₂MoO₄printed capacitors tested at room temperature, 1 kHz Substrate PressingDielectric Pressing Type Conditions Capacitance Tan δ Constant Aid NiFoil 100 MPa, 15   52 pF 0.012 4.4 paper min. @ 120° C. Ni Foil 100 MPa,15   47 pF 0.025 3.6 PFTE min. @ 120° C. PET 100 MPa, 12 17.1 pF 0.0065.0 Glossy min. @ 120° C. paper

By this example, we demonstrated fabrication of printed Li₂MoO₄capacitor structures on both Nickel foil and PET film by a coldsintering process. With conventional processing methods, where thesintering of the Li₂MoO₄ would take place at 540° C., this would beimpossible because the Nickel foil would oxidize in air at temperaturesabove 300° C., and the PET film would thermally degrade at temperaturesof about 225° C. to 260° C. Further, this example shows cold sinteringprocesses of the present disclosure can accommodate flexible substrates,and forming structures of many different inorganics sintered thereon.The relative density of the sintered ceramic on the substrate in theseexamples is estimated to be no less than about 90% based on theperformance of the prepared capacitors relative to conventionallyprepared capacitors.

Cold sintered printed LizMoat capacitors have electrical andmicrostructural properties that are similar to those that have beenconventionally processed. The ability to co-process incompatiblematerials systems, such as low temperature polymers with hightemperature ceramics, allows production of variety of new composites fordevice construction. Moreover, energy and time savings by employing thecold sintering method are significant when compared to conventionalsintering methods.

FIG. 10 is a plot comparing relative density to sintering temperature ofinorganic materials (example BaTiO₃) made by different processes. Thedifferent processes include: Conventional Sintering (CS), Two-StepSintering (TSS), Rate-Controlled Sintering (RCS), Spark Plasma Sintering(SPS), Microwave Sintering (MVS), High Pressure Sintering (HPS), FlashSintering (FS), Combined=RRS (Rapid-Rate Sintering)-RCS-LP (LowPressure)-TSS, and Cold Sintering Processes (CSP) in accordance with thepresent disclosure. A theoretical density of 6.02 g/cm3 is adopted forBaTiO₃. GS=Grain Size. As shown in the plot, cold sintering processes ofthe present disclosure are capable of fabricating relatively denseinorganic materials such as ceramics at far lower temperatures thanconventional fabrication methods.

Only the preferred embodiment of the present invention and examples ofits versatility are shown and described in the present disclosure. It isto be understood that the present invention is capable of use in variousother combinations and environments and is capable of changes ormodifications within the scope of the inventive concept as expressedherein. Thus, for example, those skilled in the art will recognize, orbe able to ascertain, using no more than routine experimentation,numerous equivalents to the specific substances, procedures andarrangements described herein. Such equivalents are considered to bewithin the scope of this invention, and are covered by the followingclaims.

1-20. (canceled)
 21. A material, comprising: at least one inorganiccompound; the material being a sintered material formed via a sinteringprocess comprising: combining the at least one inorganic compound with asolvent to form a mixture; and applying pressure and heat to the mixtureto evaporate the solvent and densify the at least one inorganic compoundto form the material, wherein the applied heat is at a temperature of nomore than 200° C. above the boiling point of the solvent, and whereinthe material is densified to a relative density of greater than 80%. 22.The material of claim 21, wherein the at least one inorganic compoundhas a particle size of less than 50 μm or less than 30 μm when combinedwith the solvent to form the mixture.
 23. The material of claim 1,wherein the combining of the at least one inorganic compound with thesolvent to form the mixture includes combining the at least oneinorganic compound with at least one other substance with the solvent toform the mixture.
 24. The material of claim 23, wherein the othersubstance is a polymer.
 25. The material of claim 21, wherein thesolvent at least partially solubilizes the at least one inorganiccompound.
 26. The material of any of claim 21, wherein the material is acomposite material.
 27. The material of claim 21, wherein a time periodof less than 180 minutes, a time period of no more than 60 minutes or atime period of no more than 30 minutes is utilized to obtain therelative density of greater than 80%.
 28. The material of claim 21,wherein the solvent includes one or more of a C₁₋₁₂ alcohol, ketone,ester, or water, or an organic acid or mixtures thereof wherein thesolvent has a boiling point below 200° C.
 29. The material of claim 21,wherein the pressure is a uniaxial pressure.
 30. The material of claim21, wherein the sintering process also includes: forming the mixture ontwo or more substrates and laminating the two or more substrates withthe material.
 31. A material, comprising: a ceramic deposited on asubstrate, the ceramic deposited on the substrate via a sinteringprocess comprising: depositing the ceramic on the substrate; exposingthe ceramic deposited on the substrate to a solvent to form a wetteddeposited ceramic; and applying pressure and heat to the wetteddeposited ceramic to evaporate the solvent and sinter the ceramic on thesubstrate, wherein the applied heat is no more than 200° C., the appliedpressure is no more than 5,000 MPa and the ceramic is sintered to arelative density of no less than 85%.
 32. The material of claim 31,wherein the sintering process also includes: forming the mixture on twoor more substrates and laminating the two or more substrates with thematerial.
 33. A device comprising: a sintered material, the sinteredmaterial comprising one of: (a) at least one inorganic compound, thesintered material being formed via a sintering process comprising:combining the at least one inorganic compound with a solvent to form amixture; and applying pressure and heat to the mixture to evaporate thesolvent and densify the at least one inorganic compound to form thematerial, wherein the applied heat is at a temperature of no more than200° C. above the boiling point of the solvent, and wherein the materialis densified to a relative density of greater than 80%; and (b) aceramic deposited on a substrate, the ceramic deposited on the substratevia a sintering process comprising: depositing the ceramic on thesubstrate; exposing the ceramic deposited on the substrate to a solventto form a wetted deposited ceramic; and applying pressure and heat tothe wetted deposited ceramic to evaporate the solvent and sinter theceramic on the substrate, wherein the applied heat is no more than 200°C., the applied pressure is no more than 5,000 MPa and the ceramic issintered to a relative density of no less than 85%.
 34. The device ofclaim 33, the device configured as a capacitor.
 35. The device of claim33, the device configured as a battery.
 36. The device of claim 33, thedevice configured as an energy-conversion system or an electrochemicalsystem.
 37. The device of claim 33, the device configured as a microwavedevice.
 38. The device of claim 33, the device configured as ceramicpackaging or electronic packaging.
 39. The device of claim 33, thedevice configured as a ceramic filter or a catalytic support.